M&R 5.1 Effects of electical signals - ligand gated ion channels

Question 1

Name 3 types of voltage-gated ion channels located in an axon membrane

Answer 1

Voltage gated Na+ & K+ channels (along the axonal membrane)
Voltage-gated Ca2+ channels (concentrated at the nerve terminal)

Question 2

Describe the general effects of depolarisation on voltage-gated Ca2+ channels and the consequences

Answer 2

Depolarisation opens VGCCs
Ca2+ enters the nerve terminal
Increased [Ca2+]i triggers the release of neurotransmitters

Question 3

What are the intracellular and extracellular concentrations of Ca2+ in an axon?
What is ECa ?

Answer 3

Intracellular = 1x10-7M 
Extracellular = 1x10-3M

ECa = +122mV

Question 4

is the structure of VGCCs most similar to that of VGNCs or VGKCs?

Answer 4

VG Na+ channels

Both have a single alpha subunit with 4 ‘quarters’

Question 5

Where are L-type Ca2+ channels found?

Answer 5

Muscle (skeletal, smooth, cardiac) , neurones and lung

Question 6

Which group are specific blockers of L-type Ca2+ channels?

Answer 6

Dihydropiridines (e.g. nifedipine)

Question 7

What does phosphorylation of Ca2+ channels do?

Answer 7

Makes the channel open more at any given voltage - therefore increases permeability to Ca2+

Question 8

In the NMJ, where are the Ca2+ channels located? Why?

Answer 8

Close to vesicle release sites
So an increase in [Ca2+]i following an AP reaching the terminal can activate proteins associated with vesicles, to promote exocytosis of ACh

Question 9

Describe the process (and proteins involved) in transmitter release at the NMJ

Answer 9

Ca2+ enters via VGCCs
Ca2+ binds to synaptotagmin, which brings the vesicle close to the membrane
SNARE complex makes a fusion pore between vesicle and membrane
ACh released via fusion pore into synaptic cleft

Question 10

In the NMJ, what receptor does ACh bind to on the post-junctional membrane? What type of receptor is this?

Answer 10

Nicotinic ACh receptor (nAChR)

= a ligand-gated ion channel

Question 11

How do nAChRs work?

Answer 11

Each nAChR has 2 binding sites for ACh (on the 2 alpha subunits)
When both have bound ACh, a conformational change causes the ion channel to open

The channel is a non-specific cation channel - allows Na+ and K+ through [Na+ enters, K+ leaves]

Question 12

If NMJ nAChRs let both Na+ and K+ through, then why does their activation result in depolarisation?

Answer 12

The RMP of skeletal muscles is ~85-90mV
Therefore RMP is close to Ek, so there is little driving force for K+ to leave
RMP is far from ENa, so there is a big driving force for Na+ to enter
Therefore Na+ entry will overcome K+ exit = net inward flow of positively charged ions = depolarisation

Question 13

What is the depolarisation produced by the nAChR after ACh binding called?
What does it do?

Answer 13

An end-plate potential (EPP)

It in turn raises the muscle above threshold so that VGNCs are activated and an AP is produced in the muscle membrane

Question 14

What is the ‘reversal potential’ of an ion channel?

Answer 14

The point at which there is no net movement across the membrane

Question 15

How is the amplitude of an EPP related to calcium concentration? What does this prove?

Answer 15

The EPP amplitude decreases as Ca2+ concentration is decreased

Proves that Ca2+ release is a necessary step for transmitter release

Question 16

How is synaptic transmission terminated in the NMJ?

Answer 16

Degradation of ACh by acetylcholinesterase (AChE)

Question 17

Name a competitive blocker of nAChRs

Answer 17

d-Tubocurarine

Question 18

Name a depolarising blocker of nAChRs

Answer 18

succinylcholine

Question 19

How does d-tubocurarine block nAChRs?

Answer 19

Competitive block - it sits in ACh’s binding sites which prevents ACh from binding.
It doesn’t cause the conformational change, so the channel remains closed and no EPP is produced.

Question 20

How does succinylcholine block nAChRs?

Answer 20

Depolarising block:
(normally brief depolarisation at the post-junctional membrane activates adjacent Na+ channels by local spread of charge, causing a muscle AP)

Succinylcholine causes a prolonged depolarisation. This causes adjacent Na+ channels to become inactivated, so they can no longer be activated to cause a muscle AP

Question 21

Can competitive block by tubocurarine be overcome?

Answer 21

Yes - block can be overcome if lots of ACh around - displaces curare

Question 22

Can depolarising block by succinylcholine be overcome?

Answer 22

No
Even if some ACh came along and managed to bind with some free receptors & depolarise further, this would not be able to trigger the AP because of inactivated Na+ channels

Question 23

What is a miniature end-plate potential (MEPP)?

Answer 23

A small depolarisation , due to the spontaneous release of a vesicle resulting in its package of ACh being released

Always similar size (~1mV) because vesicles are similar sizes
don’t reach threshold or activate AP

Question 24

What is Myasthenia Gravis?

Answer 24

An autoimmune disease where patients suffer profound muscle weakness, which increases with exercise

Question 25

Which receptors are affected in Myasthenia gravis? How?

Answer 25

Antibodies are directed at the nAChRs on the post-synaptic membrane of skeletal muscle
This leads to loss of functional nAChR by complement-mediated lysis and receptor degradation

Question 26

How are EPPs affected in Myasthenia gravis? Why?

Answer 26

EPPs are reduced in amplitude
Because although the same amount of ACh is released per vesicle, the response is smaller because nAChRs have been damaged

Question 27

What class of drug can be used to treat both Myasthenia gravis and d-tubocurarine toxicity? Why?

Answer 27

Acetylcholinesterase inhibitors

Because in both conditions the availability of functioning nAChR receptors is reduced. The AChE inhibitor blocks ACh degradation, therefore raising the amount in the NMJ. The extra ACh which collects can then correct for the lack of functioning nAChRs by activating the remaining normal receptors at a higher rate.

Question 28

__________ - __________ ________ is the mechanism by which an AP in a muscle fibre leads to contraction of the fibre

Answer 28

Excitation-contraction coupling